Prevention of ice formation by applying electric power to a liquid water layer

ABSTRACT

A first electrode is separated from a second electrode by an interelectrode space. The interelectrode space does not exceed 3 mm, and preferably does not exceed 100 μm. Liquid water fills the interelectrode space, thereby electrically connecting the first electrode and the second electrode. A power supply, preferably low-frequency AC, is connected to the first and second electrodes, generating a current through the water in the interelectrode space. The applied electric power prevents freezing of a thin liquid water layer in the interelectrode space, thereby preventing ice formation.

RELATED APPLICATIONS

This application claims the benefit of U.S. provisional applicationsSer. No. 60/262,775, filed Jan. 19, 2001, and Ser. No. 60/283,670, filedApr. 12, 2001. This application is also a continuation-in-partapplication of commonly-owned and co-pending U.S. patent applicationSer. No. 09/426,685, filed Oct. 25, 1999, which is a divisionalapplication of U.S. patent application Ser. No. 09/094,779, filed Jun.15, 1998, issued as U.S. Pat. No. 6,027,075 on Feb. 22, 2000; and thisapplication is also a continuation-in-part application of commonly-ownedand co-pending PCT application PCT/US00/35529, filed Dec. 28, 2000,which claims the benefit of U.S. provisional application Ser. No.60/173,920, filed Dec. 30, 1999, now abandoned; each of which relatedapplications is hereby incorporated by reference.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has certain rights in this invention as provided forby the terms of Grant #DAAH 04-95-1-0189, awarded by the Army ResearchOffice, and of Grant No. MSS-9302792, awarded by the National ScienceFoundation.

FIELD OF THE INVENTION

The invention is related to the field of ice prevention, specifically,to preventing formation of ice on surfaces of solid objects.

BACKGROUND OF THE INVENTION

Statement of the Problem

Ice causes problems on many various kinds of surfaces. For example, iceaccumulation on aircraft wings endangers the plane and its passengers.Accumulations of ice formed by the condensation and freezing of water onthe outside surfaces of heat exchangers in freezers reduces heattransfer efficiency and often results in physical damage to coolingcoils. Problems associated with ice are particularly obvious withrespect to land-based transportation systems, including automobiles,trailers, trolleys and railroads. Ice on automobile windshields andwindows decreases driver visibility and safety. Removing ice fromwindshields is a recurring chore that is often unsatisfactorilyperformed. Ice on roads is frequently a cause of automobile accidentsresulting in personal injury and death, as well as material damage. Iceon airport runways causes delays in air traffic. Large amounts ofmaterial resources, money and man-hours are spent annually to remove iceand snow from roads, sidewalks and bridges to reduce risks of slippingand skidding on iced surfaces.

Conventional resistive heating systems to remove ice and snow have high,sometimes economically unfeasible, power requirements. Application ofchemical agents to remove ice has temporary effects, is limited torelatively small surface areas, and is labor and equipment intensive.Also, once ice has formed on surfaces, it may be difficult to remove.Also, some applications require the prevention of ice in the firstplace. Growth of even 1 mm of ice at certain locations on airplane wingsand helicopter blades seriously decreases their performance. Otherexamples include ice growth on windshields of airplanes and automobilesand on the outside of freezer coils, which have a low level of toleranceto even thin layers of ice.

SUMMARY OF THE INVENTION

The invention helps to solve some of the problems mentioned above byproviding systems and methods for preventing the freezing of a liquidwater layer and the formation of ice. In preferred embodiments, AC poweris utilized, instead of DC power, thereby avoiding some of thedisadvantages associated with DC power and utilizing readily availableAC power. Systems and methods in accordance with the invention areapplicable to problems associated with ice and surfaces in many diversefields.

A first basic embodiment in accordance with the invention includes: afirst electrode layer disposed on the surface of an object beingprotected against ice formation; a second electrode layer proximate tothe first electrode layer. The first and second electrodes are separatedby an interelectrode distance. The first electrode and the secondelectrode define an interelectrode space between the electrodes. An ACpower source is connected to the first electrode and the secondelectrode. When conductive water fills the interelectrode space betweenthe electrodes, the water completes an electrical circuit including thetwo electrodes. Preferably, the power source provides an AC voltagehaving a frequency in a range of from 15 Hz to 1 kHz. The interelectrodedistance generally does not exceed 3 mm. Typically, the interelectrodedistance does not exceed 500 μm (“microns”, micrometers). Preferably,the interelectrode distance does not exceed 100 μm, and most preferably,it does not exceed 10 μm. When the thickness of the interelectrodedistance is very small, that is, not greater than 100 μm, the voltage ofthe AC power source is generally in a range of from 0.1 to 100 volts,preferably in a range of from 5 to 25 volts. When the interelectrodedistance does not exceed 100 μm, then suitable prevention of iceformation is typically achieved when the current density in a liquidwater layer in the interelectrode space is in a range of from 1 to 100mA/cm².

Electrodes in accordance with the invention have various shapes. Forexample, a first electrode or a second electrode may have the shape of,among others, a point or sphere, a line, a strip or a surface-conforminglayer.

In one variation of a first basic embodiment, a first electrode layer, asecond electrode layer, and a porous insulator layer, between the firstand second electrode layers, form a multilayer laminate disposed on thesurface of the object being protected against ice. In a second variationof the first basic embodiment, a first electrode layer and a secondelectrode layer are disposed side-by-side on the surface of the object,separated by the interelectrode space. In this variation, the firstelectrode layer and the second electrode layer typically areinterdigitated, that is, a plurality of “fingers” of the first electrodelayer are disposed on the surface alternately with a plurality of“fingers” of the second electrode layer, each of the alternating fingersseparated from adjacent fingers by an interelectrode space. In a secondbasic embodiment of the invention, a DC power source is utilized insteadof an AC power source.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the invention may be obtained byreference to the drawings, in which:

FIG. 1 depicts an embodiment in accordance with the invention suitablefor preventing formation of ice on a surface by preventing freezing of aliquid water layer;

FIG. 2 depicts an exemplary multi-layer laminate in accordance with theinvention disposed on an aerofoil;

FIG. 3 contains a graph in which leading-edge temperature, T, in ° C.,and electric power dissipation, P, in units of W/cm², are plotted as afunction of time, t, in seconds, in the exemplary system depicted inFIG. 2;

FIG. 4 depicts an exemplary system having interdigitated electrodes;

FIG. 5 depicts a schematical cross-sectional view of a portion of asystem containing interdigitated electrodes; and

FIG. 6 is a graph in which relative electrical conductivity is plottedas a function of location between electrodes, spaced approximately 3 mmapart.

DESCRIPTION OF THE INVENTION

The invention is described herein with reference to FIGS. 1-6. It shouldbe understood that the structures and systems depicted in schematic formin FIGS. 1, 2, 4 and 5 serve explanatory purposes and are not precisedepictions of actual structures and systems in accordance with theinvention. Furthermore, the embodiments described herein are exemplaryand are not intended to limit the scope of the invention, which isdefined in the claims below.

The diagram of FIG. 1 depicts a preferred embodiment of a system 100 inaccordance with the invention. In system 100, solid object 102 has anexterior surface 104 on which ice formation is prevented by preventingfreezing of a liquid water layer. A laminate structure 106 comprising aplurality of layers 110, 112 and 114 is disposed on surface 104. A firstelectrode layer 110 is located at surface 104. A porous insulator layer112 is disposed on first electrode layer 110, and a second electrodelayer 114 is located on insulator layer 112, covering insulator layer112, first electrode layer 110 and surface 104. Second electrode layer114 has an outside surface 115, exposed to water, such as precipitationor condensation water. Continuously porous insulator layer 112 createsan interelectrode space 118 and is porous to water. System 100 furthercomprises an AC power source 120 electrically connected to firstelectrode layer 110 and second electrode layer 114 by connector 122 andconnector 124, respectively. In system 100, second, top electrode layer114 and insulator layer 112 are porous to water. As a result, when waterdeposits on outside surface 115, it fills the pores throughout secondelectrode layer 114 and the interelectrode space 118 of insulator layer112, thereby creating a liquid water layer 119 (designated by the solidcircles interspersed in interelectrode space 118 and second electrodelayer 114 of FIG. 1) that electrically connects first electrode layer110 and second electrode layer 114.

It is a feature of a system and a method in accordance with theinvention that the interelectrode distance between the first electrodeand the second electrode is small. Generally, the larger theinterelectrode distance, the greater the applied voltage (whether AC orDC) must be to provide sufficient power for preventing freezing of waterin the liquid water layer. Based on empirical data contained in theexamples described below, one skilled in the art may calculate a voltagecorresponding to any thickness of interelectrode distance. Nevertheless,practical considerations generally limit the interelectrode distance toa value less than 3 mm, more typically to a distance not exceeding 500μm, preferably not exceeding 100 μm, and most preferably not exceeding10 μm. There is no theoretical lower limit to the size of theinterelectrode distance. Generally, the interelectrode distancecorresponds to the thickness of the interelectrode space between thefirst electrode and the second electrode. In FIG. 1, the size of theinterelectrode distance is defined by the thickness of porous insulatorlayer 112, located between first electrode layer 110 and secondelectrode layer 114. With currently utilized conventional depositiontechniques, electrodes and porous insulator layers may be formedroutinely to achieve interelectrode distances as small as 5 nm(“nanometer”).

As discussed above, the power required in a system in accordance withthe invention is highly dependent on the interelectrode distance, withthe required voltage decreasing as interelectrode distance decreases.When the interelectrode space does not exceed 100 μm, power source 120is suitable when it provides a voltage bias in the range of from 1 to100 volts, preferably in a range of from 5 to 25 volts. In embodimentshaving an interelectrode space not exceeding 100 μm, a voltagegenerating a current density in water in the interelectrode space 118 ina range of from 1 to 100 mA/cm², or greater, typically preventsformation of ice down to about −10° C.

In a method in accordance with the invention, when water has depositedon outside surface 115 of laminate 106, it permeates porous secondelectrode 114 and insulator layer 112, forming a liquid water layer 119.Liquid water layer 119 in interelectrode space 118 electrically connectsfirst electrode layer 110 and second electrode layer 114. When a voltagebias is applied to electrodes 110 and 114, it inhibits ice formation inliquid water layer 119 in the interelectrode space 118, especially nearelectrodes 110, 114. Presence of even a thin layer of liquid watercovering surface 104 of solid object 102 prevents any significantformation of ice on surface 115, even at cold temperatures well belowthe freezing point of water. Consumption of electrical power to preventice formation in a liquid water layer in accordance with the inventionis typically in a range of from 0.75 to 10 kW/m². The greater theinterelectrode distance is, the greater the applied voltage (whether ACor DC) must be to generate electric power sufficient to prevent iceformation.

A system 100 and a corresponding method are useful in many types ofcircumstances and for protecting many types of solid objects against iceformation. For example, solid object 102 includes: an airplane wing; ahelicopter blade; a heat exchanger coil; roads and sidewalks;windshields and windows; and many others.

A nonexclusive list of materials which may be contained in the firstelectrode or second electrode layers includes: aluminum, copper,titanium, platinum, nickel, gold, mercury, palladium and mixturesthereof. Other suitable conductive electrode materials include carbonand conductive metal oxides, such as SnO₂, InSnO₂, RuO₂ and IrO₂. Aspecific advantage of embodiments in accordance with the invention inwhich AC power is applied to the electrodes is that the electrodes maycomprise substantially corrosion-resistant titanium. Titanium isvirtually useless as electrode material when used with DC voltage. Incontrast, titanium functions well as an electrode when the AC voltagehas a frequency of 50 Hz or greater.

As depicted in FIG. 2, and described below in Example 1, the secondelectrode may be a second electrode layer in the form of a mesh havingelectrically conductive mesh fibers. Typically, mesh fibers comprise aconductive metal. Preferably, the mesh fibers have a thickness in arange of from 1 to 50 μm. Alternatively, the second electrode layer, aswell as the first electrode layer, may be formed using standardtechniques, such as sputtering, chemical vapor deposition, spraying,painting, photolithography, electroplating and others.

In embodiments in which a porous insulator layer is disposed between thefirst electrode layer and the second electrode layer, forming theinterelectrode space, the porous insulator layer has a total volume anda pore space, and the pore space may occupy between 0 and 100 percent ofthe total volume. Typically, the pore space occupies about 50 percent ormore of the total volume, and preferably about 70 percent. Frequently,the first electrode layer comprises aluminum and the porous insulatorlayer comprises aluminum oxide. In such embodiments, the porousinsulator layer typically comprises anodized aluminum. A first electrodelayer comprising aluminum is formed on the surface of the object beingprotected using any of a variety of standard methods. Then, the uppersurface of the first electrode layer is anodized using standardtechniques. The anodized aluminum can be made to be porous.

Frequently, the surface of the solid object is conductive or includesthe first electrode layer. For example, the conductive outer surface ofan airplane wing may function as a first electrode layer in a system inaccordance with the invention.

In some embodiments in accordance with the invention, as depicted inFIG. 4 and described below in Example 2, both the first and the secondelectrode layers are disposed side-by-side on the surface of the objectbeing protected, separated by an interelectrode distance correspondingto the interelectrode space. In such embodiments, the first and secondelectrodes are typically interdigitated.

Terms of orientation, such as “top”, “bottom”, “above” and others, areused with relation to the surface being protected against ice formation.For example, with reference to FIG. 1, bottom electrode layer 110 iscloser to surface 104 of object 102 than top electrode layer 114.Therefore, top electrode layer 114 is “above” bottom electrode layer110. In FIG. 1, surface 104 is substantially horizontal. The term“cover” indicates that a first element that covers a second element isabove the second element. For example, in FIG. 1, top, second electrode114 and covers both bottom, first electrode 110 and surface 104. It isunderstood that a surface being protected in accordance with theinvention may be spatially oriented in many positions different fromhorizontal.

In a second basic embodiment of systems and methods in accordance withthe invention, a DC power supply is used to provide a DC voltage acrossthe first and second electrodes. Otherwise, a DC system and method arevirtually the same as described above with reference to FIG. 1, or belowwith reference to FIGS. 4 and 5. For example, a DC voltage was utilizedin Examples 1-4, described below. Although the first, bottom electrodelayer 110 of FIG. 1 may serve either as cathode or anode in the DCsystem, typically, first electrode layer 110 of laminate 106 functionsas cathode.

EXAMPLE 1

A system 200 in accordance with the invention is depicted in FIG. 2. Analuminum foil 210 was stretched over a massive cylindrical aluminumaerofoil 202. A porous insulating film 212 of aluminum oxide, Al₂O₃,having a thickness of approximately 10 μm was formed by anodizingaluminum foil 210. Then, a stainless steel mesh 214 electroplated withnickel and platinum was disposed on the aluminum oxide, forming athree-layer laminate 216. Mesh 214 was a 400-gauge mesh woven of 22.4 μmwire. Two thin thermocouples were inserted in the aluminum cylinder at 2mm and 25 mm from the mesh. Cold air having a temperature of −12° C. anda water content of zero and then 0.25 g/m³ was flowed at a speed of 200miles/hour across aerofoil 202 covered by laminate 216. A DC powersupply having a bias of 50 volts was connected to electrodes 210, 214during the whole time that measurements were taken. Aluminum foil (firstelectrode) 210 functioned as the cathode and mesh (second electrode) 214functioned as the anode.

In the graph of FIG. 3, leading-edge temperature, T, in ° C., andelectric power dissipation, P, in the mesh electrodes, in units ofW/cm², are plotted as a function of time, t, in seconds, during whichwater supply in the cold air was turned “on”. The data show that beforethe water supply was turned on, the temperature measured in the mesh wasapproximately −8° C., and the power consumption was approximately zero.Even while the power consumption was zero, the mesh temperature exceededthe ambient air temperature due to adiabatic compression of the air atthe leading-edge of the aerofoil. At a time of approximately 350seconds, the water supply through a misting apparatus was turned “on”. Astream of micron-size liquid water particles was thereby injected at arate of 0.25 g/m³ into the cold air stream flowing across the aerofoil.Because the misted water particles were so small, they quickly reachedtemperature equilibrium of −12° C. with the cold air. On the other hand,the water particles remained liquid at −12° C. because of the largeincrease in surface energy that would be required to change theparticles from the liquid state to a frozen solid state. As theair-borne particles deposited on the surface of the aerofoil, liquidwater filled the pores in mesh electrode 214 and the interelectrodespace of aluminum oxide layer 212, thereby forming a liquid water layer,which electrically connected aluminum foil electrode 210 and wire meshelectrode 214. As a result, the power consumption suddenly rose to about0.7-0.9 W/cm². Also, the measured temperature rose from about −8° C. toabout −5° C. The increase in temperature was probably a result of therelease of energy as water vapor in the air condensed on the aerofoil.Nevertheless, at no time did liquid water freeze on the surface of theaerofoil even though its measured temperature was −5° C. At a time, t,of approximately 830 seconds, the water supply to the air was turned“off”. Thereafter, the water in the mesh and the interelectrode space ofthe aluminum oxide layer quickly drained off from the leading-edge wherethe thermocouples were located or evaporated into the dry air. As aresult, the electrical connection between the aluminum electrode and theupper mesh electrode was interrupted, causing the power consumption todecrease rapidly from the value of about 1 W/cm² to almost zero, eventhough the voltage of the power source was not decreased.Simultaneously, the measured temperature dropped from about −4° C. toabout −8° C. The power and temperature curves in FIG. 3 show that whilewater was supplied to the cold air stream, the liquid water in theliquid water layer on the aerofoil did not freeze and form ice, eventhough the water was supercooled to a temperature value of at least lessthan −5° C., during which time power was consumed at a rate of about0.7-0.8 W/cm². As explained below in Example 3, the measured electricpower consumption would have been insufficient to raise the temperatureof the bulk water through resistive heating by even 0.1° C.

Similar prevention of ice formation was observed in exemplary systems inaccordance with the invention when the mesh was covered with a thinliquid water layer and the system was cooled by circulating coolant.

EXAMPLE 2

A system as described in Example 1 was assembled, but the porousinsulating film of aluminum oxide had a higher pore density, that is,the pore space occupied approximately 70 percent of the total volume ofthe insulating film. As a result, the water-filled channels of theliquid water layer in the porous insulating film provided higherconductivity (that is, less electrical resistance) between theelectrodes, thereby allowing higher current density for a given voltage.Measurements similar to those in Example 1 were made. The same degree ofice prevention as in Example 1 was achieved by using only 20-25 volts,instead of 50 volts. The lower voltage corresponded to an electric powerof just 0.3 W/cm², instead of approximately 0.8 W/cm² of Example 1.

EXAMPLE 3

Interdigitated copper-grid electrodes were electroplated with gold onthe surface of a 125 μm thick kapton film. FIG. 4 shows a top view inschematic form of a system 400 in accordance with the invention havinginterdigitated electrodes 410. FIG. 5 shows a cross-section section 455of system 400, indicated by lines 5 in FIG. 4. As depicted in FIGS. 4-5,a first portion of kapton film surface 404 of kapton film 402 wascovered with first electrode stem 419 and corresponding first electrodes420. A second portion of kapton film surface 404 was covered with secondelectrode stem 421 and corresponding second electrodes 422. A thirdportion 426 of the electrically nonconductive kapton film surface 404was not covered with electrode material. Adjacent “fingers” ofinterdigitated electrodes 410, functioning alternately as anodes 420 andcathodes 422, were separated by interelectrode spaces 426.Interdigitated electrodes 410 each had a width of about 50 μm; athickness of the metal electrodes was approximately 2 μm. Interelectrodespaces 426 each had a width of about 50 μm. The resulting grid ofinterdigitated electrodes 410 and interelectrode spaces 426 covered anarea on surface 404 of approximately 5 cm×5 cm. A DC power sourceproviding a voltage bias of 5 volts was attached to the anodes andcathodes. Kapton film 402, containing electrodes 410 and interelectrodespaces 426, was covered with about 1 mm of water. As depicted in FIG. 5,water film 460 covered kapton film 402 and interdigitated electrodes 410and filled interelectrode spaces 426. A voltage of 5 volts was applied,and then the system was cooled to −10° C. A current density of about 1mA/cm² was measured in the water between selected anodes and cathodes.As long as the voltage was continuously applied, a liquid water layer462 directly adjacent to the electrodes 410 did not freeze, while anupper ice layer 464 of water film 460 formed as a result of the freezingtemperature. The unfrozen liquid water layer 462 had a thicknessestimated optically as 5 to 25 μm. The temperature of the substrate film402 and the ice 464 above the electrodes 410, as measured with the thinthermocouples, was maintained at −10° C. A simple calculation showedthat the low electric power density of ≦0.5 mW/cm² could have warmed theliquid bulk of liquid water layer 462 by only about 0.05° C. aboveambient temperature by resistive heating. Power was turned “off′,resulting in complete freezing of liquid water layer 462. After freezingoccurred, a voltage bias was applied to the electrodes again, but at ahigher voltage in order to provide the same electric power as beforefreezing (the electrical conductivity of the ice is less than liquidwater, so a higher voltage is required in the ice to provide the samepower as in water). The application of the same electric power to theice that had prevented freezing of water in liquid water layer 462,however, did not cause melting of the layer of ice. This indicates thatthe voltage applied to the electrodes in the liquid water systemprevented ice formation in accordance with the invention, but the sameelectric power was unable to melt ice through heating after the ice hadformed. It should be noted that the thickness of 2 μm of the metalelectrodes in Example 3 was larger than necessary to provide sufficientelectric current through the liquid water layer.

Physical Mechanisms

It is believed that one or both of two different physical mechanismsexplain the results of Examples 1-3, specifically, and the function ofembodiments in accordance with the invention, generally.

According to the first prospective physical mechanism, an embodiment inaccordance with the invention prevents formation of ice throughlocalized resistive heating and melting of ice crystals during initialice nucleation in supercooled water. Accordingly, in supercooled waterbetween electrodes, a very thin layer of ice starts to grow in the bulkwater of liquid water layer between electrodes, or as is more common, atthe electrode, where water has a greater tendency to nucleate into icecrystals at the solid surface of the electrode. A current passingthrough the liquid water layer between electrodes also passes throughthe ice layer. Ice has an electrical resistivity that is from 2 to 4orders of magnitude greater than the resistivity of water. See Physicsof Ice, V. Petrenko and are R. Whitworth, Oxford University Press(1999). As a result, when even a thin layer of ice appears, most of theelectrical power dissipates in the ice, rather than in the liquid water.When a layer of ice appears, the heat produced by electric currentpassing through the highly resistive ice melts the ice in its nucleationstage, even in a supercooled liquid water. Thus, electric currentactually prevents ice nucleation. Calculations show, for example, thatif liquid water is supercooled to −10° C., and if a layer of ice havinga thickness of 10 μm forms, then a current having a current density ofapproximately 13 mA/cm² melts the ice. This is similar to the currentdensity that prevented freezing and ice formation in Example 1. Thismechanism succeeds in maintaining the liquid state of even supercooledwater because it prevents development of ice seeds during nucleation.Ice cannot form without ice seeds. On the other hand, once ice hasformed beyond the nucleation stage, it is not possible to melt the iceusing the same level of power described here to prevent the ice. Also,to keep the water above its melting temperature by heating it requiresmuch more power, that is, from 10 to 100 times more power than it doesto prevent formation of ice by melting ice seeds in their nucleationstage.

Therefore, the term “prevention of ice” and similar related terms whenused in this specification have a qualified meaning. In view of theforegoing discussion concerning heating and melting of ice seeds duringice nucleation, it is clear that relatively small amounts of icecrystals may form in a system and a method in accordance with theinvention, before being heated and melting. Thus, the term “preventionof ice” means that the bulk of the liquid water in a liquid water layerin and around the interelectrode space does not freeze, and that a thinliquid water layer in the interelectrode space and in the regionsimmediately adjacent to the electrodes remains liquid. The term“prevention of ice” and related terms also include a broader meaning.They also refer to inhibiting the formation of significant ice depositson a liquid water layer. Because it is very difficult for ice to collectand remain on a liquid water surface, a liquid water layer in accordancewith the invention disposed on a solid surface, such as an airplane wingor a windshield, effectively prevents the growing and adhesion of solidice on the surface. Similarly, the term “prevent freezing of liquidwater” and related terms means that the liquid water layer remainssubstantially unfrozen, although minute volumes of water may freezeduring initial nucleation before melting in accordance with theinvention. In a system and method in accordance with the invention, icemay form in regions not included in the liquid water layer in and aroundthe interelectrode space. For example, ice may form or be deposited onupper surface 115 of second electrode 114 of FIG. 1. Development ofsignificant deposits of ice contiguous with the liquid water layer doesnot occur, however, because of insufficient adhesion strength betweenice and the liquid water layer. The term “supercooled water” has itsusual technical meaning of liquid water having a temperature lower thanits freezing temperature. The term “liquid water layer” is used in abroad sense in this specification. Its meaning includes a continuousvolume of liquid water capable of electrically connecting twoelectrodes, in electrical contact with the electrode, and capable ofcarrying an electrical current. A “liquid water layer” may be anyuninterrupted volume of water, that is, uninterrupted by non-watermaterials. Or, for example, a “liquid water layer” may comprise aplurality of water channels interspersed in a non-water material, forexample, in a porous insulator layer, as described above. The liquidwater layer is not necessarily confined to the boundaries of aninterelectrode space. For example, in FIG. 1 a liquid water layeroccupies the pore space of porous insulator 112 and the pore space ofporous second electrode 114. In system 400 depicted in FIG. 4, a liquidwater layer covered the whole surface area containing interdigitatedelectrodes 410 and interelectrode space 426. The term “interelectrodedistance” refers to the shortest distance between the first electrodeand the second electrode of a system in accordance with the invention.Embodiments in accordance with the invention are described herein withreference mainly to electrode layers. In other embodiments in accordancewith the invention, however, functional electrodes have non-layershapes, such as, sphere-shaped or wire electrodes. Thus, the term“interelectrode space” refers generally to a region located between thefirst and second electrodes. For example, interelectrode space 118 shownin the cross-sectional view of system 100 in FIG. 1 is a layer-likeregion between electrode layers 110 and 114.

In a given system under given conditions, the extent of the liquid waterlayer, that is, the region of liquid water that is prevented fromforming ice, is dependent on the amount of electric power applied. Byincreasing electric power, the volume of liquid water that does notfreeze increases. In the first proposed mechanism, the flow ofelectrical current through the liquid water layer results in heating andmelting of ice seeds in their nucleation phase before they growappreciably. Operation according to this mechanism occurs when thecurrent density reaches or exceeds a minimum level, as discussed above.Theoretically, the current may be generated from any source. Forexample, a capacitive AC current generated in a high-frequencyalternating electromagnetic field can provide sufficient current toprevent ice formation in accordance with the invention. Practically,however, in preferred embodiments, as described in detail in thisspecification, a liquid water layer provides electrical connectionbetween two electrodes, and the electrical power source provides avoltage across the electrodes, which generates the electrical current.Either AC or DC current melts ice seeds in their nucleation stage inaccordance with the invention. AC current having a frequency greaterthan about 10-15 Hz is usually preferred in order to avoid electrolyticcorrosion of electrodes.

According to the second prospective mechanism, applying a voltage to anelectrode in the liquid water layer generates intrinsic water ions thatdepress the freezing point of water, thereby preventing ice formation.

When a low-frequency (“LF”) AC voltage or a DC voltage is applied acrosstwo electrodes immersed in water, an increase in electrical conductivityof the water occurs and supercooled water remains liquid. Thesephenomena are fully reversible, and water regains its physicalproperties within about ten seconds after the voltage bias is shut“off”. The maximum effects occur very close to the electrodes. It isbelieved that these phenomena are the result of the generation ofintrinsic water ions in the bulk water resulting from migration ofchemical reaction products formed at the electrode surfaces.

The generation of H⁺ and OH⁻ ions at the electrode interfaces may berepresented according to the following reaction equations:2H₂O+2e ⁻⇄2H⁺+2OH³¹, (at cathode, or “negative” electrode)  Equation (1)and2H₂O+2e ³¹⇄O⁺+2H⁺, (at anode, or “positive” electrode)  Equation (2)where O⁺ and H⁺ are atomic oxygen and hydrogen radicals. These reactionsoccurring at the electrodes are typical water electrolysis reactions.During electrolysis, atomic hydrogen atoms recombine at the cathode,forming bubbles of molecular H₂ gas. Similarly, atomic oxygen atomsrecombine at the anode, forming bubbles of molecular O₂ gas duringelectrolysis. Another phenomenon occurs, however, which was notpreviously recognized in the art. Atomic oxygen and hydrogen radicals,O⁺ and H⁺, migrate naturally from regions of high concentration at theelectrodes into the bulk of the water between electrodes. There, in thebulk water between electrodes, those atoms similarly recombine intomolecules of oxygen and hydrogen. Each recombination event releasesabout five electron volts of energy, and that energy is sufficient tobreak several neighboring water molecules into H⁺ and OH⁻ ions. It isestimated that as many as 10 percent of the water molecules near theelectrodes may dissociate into ions; this is equivalent to an H⁺ or aOH⁻ concentration of 3.08 moles per liter. When the electrodes arerelatively far apart, as in typical electrolysis systems, theconcentration of intrinsic water ions, H⁺ and OH⁻, remainsinsignificant. When the electrodes are close together, however, inaccordance with the present invention, it is believed that the intrinsicwater ion concentration is sufficient to depress the freezing point ofthe bulk water in a liquid water layer in the interelectrode spacebetween electrodes.

A thermodynamic reason for the freezing point depression is thatdissolution of the ions in such a highly polar liquid as water lowersthe free energy of the water. These ions affect the free energy of iceto a much less degree, due to the fact that the microscopic dielectricconstant of ice is just 3.2, while in water it equals about 87 (at 0°C.). Also, impurity ions are much less soluble in ice than in water.Thus, ions decrease the free energy of water, but leave that of icealmost unchanged. Because of this, the free energy of water remains lessthan that of ice down to lower temperatures, thus delaying the phasetransition. It is believed that systems and methods in accordance withthe invention could achieve freezing point depressions in water in arange of from 1° C. to a theoretical upper limit of 80° C. The extent ofthe freezing point depression is dependent on numerous factors,including among others: the initial, pre-voltage ion-content andconductivity of the water; the interelectrode distance; the magnitude ofthe applied voltage; the frequency of the applied voltage; and thecomposition of the electrodes. The term “electrode” is used in a broadsense in this specification. The term “electrode” refers to anelectrical conductor at the surface of which a change occurs fromconduction by electrons to conduction by ions or colloidal ions. Theterm “voltage” is used in a broad sense. The term voltage may refer tothe voltage of a circuit not effectively grounded, so that it means thehighest nominal voltage available between any two conductors of anelectrical circuit. It may also refer to the voltage of a constantcurrent circuit, for which it means the highest normal full-load voltageof the current. It may refer to the voltage of an effectively groundedcircuit, that is, the highest nominal voltage available between anyconductor of the circuit and ground, unless otherwise indicated. Theterm “voltage” may also mean the effective (rms) potential differencebetween any two conductors or between a conductor and ground.

EXAMPLE 4

The relative conductivity was measured in distilled water disposedbetween two platinum electrodes. A DC bias of 5V was applied across theelectrodes. The temperature of the system was 20° C. AC conductivity, σ,of the water between the electrodes was measured at a frequency of 1kHz. The initial conductivity of the distilled water before applicationof the DC bias was σ₀=10⁻³ S/m. In the graph of FIG. 6, relativeconductivity, σ/σ₀, is plotted as a function of location between theelectrodes, which were spaced approximately 3 mm apart. The data pointsplotted in FIG. 6 show that at a distance of about 400 μm from eitherthe cathode or the anode, the relative conductivity had a value of about5. At a distance of about 100 μm from either the anode or cathode, therelative conductivity was about 10. This increase in conductivity is aresult of the increase in intrinsic ion concentration of H⁺ and OH⁻ ionsgenerated in the interelectrode space between the electrodes.

Theoretically, even a single electrode having a voltage and being incontact with a liquid water layer can generate either atomic oxygen orhydrogen radicals, which can recombine in the bulk water to formintrinsic water ions. Practically, however, in preferred embodiments inaccordance with the invention, a liquid water layer provides electricalconnection between two electrodes, and an electrical power sourceprovides a voltage across the electrodes, so that atomic radicals aregenerated at both electrodes. Furthermore, the voltage across theelectrodes generates an electric current that passes through the liquidwater layer, preventing ice formation according to the first proposedmechanism. Therefore, the terms “voltage” and “current” are usedsomewhat interchangeably in this specification. An advantage of systemsand methods in accordance with the invention is the ability to userelatively low amounts of electric power to prevent freezing in a liquidwater layer and thereby prevent ice formation. Another advantage is theability to use AC voltage. In contrast, electrolysis only works with DCvoltage or AC voltage having a very low frequency not exceeding 10-15 Hzto produce hydrogen and oxygen gas bubbles that decrease ice adhesion.In systems utilizing electrolysis to form gas bubbles, DC voltage orvery low frequency AC voltage is utilized to provide a highconcentration of hydrogen radicals, at the cathode, and oxygen radicals,at the anode, that recombine to form bubbles of molecular gas. Atfrequencies greater than 10-15 Hz, there may be insufficientaccumulation of molecular hydrogen and molecular oxygen at theelectrodes to initiate nucleation of gas bubbles on the electrodes. Insystems and methods in accordance with the invention, the polarity ofeach of the two electrodes alternates between plus and minus with thefrequency of the AC power source. At frequencies in excess of 10-15 Hz,the continuous switching between production of atomic hydrogen radicalsand atomic oxygen radicals, therefore, may not allow accumulation ofeither hydrogen or oxygen sufficient for nucleation of gaseous H₂ or O₂bubbles. The atomic oxygen and hydrogen radicals, O⁺. and H⁺., that areproduced alternately at the electrodes do, however, migrate from theelectrodes into the bulk water, where they recombine, releasing energyand thereby produce additional H⁺ and OH⁻ ions. When the electrodes arerelatively close together, it is believed that the resultingconcentration of H⁺ and OH⁻ ions produced at the electrodes and by therecombination events in the bulk water depresses the freezing point ofthe water.

The first electrode and second electrode layers may comprise anyconductive metal at which the reactions represented by Equations (1) and(2) occur. In platinum-plated electrodes, platinum functions as acatalyst for the reactions represented by Equations (1) and (2).

In preferred embodiments in accordance with the invention, an AC powersource is utilized. The frequency of the AC power source may be selectedfrom range of from 0 through the MHz range, with practically no upperlimit. In other words, in the lower end of the range, virtually DCvoltage may be applied across the electrodes. An advantage of using ACvoltage is that it is often more readily available and it inhibitscorrosion, as mentioned above. Preferably, an AC power source generatesan AC current having a frequency in a range of from 15 Hz to 1 kHz. Thepreferred lower limit of 15 Hz is the frequency at which the amounts ofelectrolysis gases collecting at the electrodes become insignificant. Asa result, the tendency of electrodes to corrode decreases significantlyabove 15 Hz. The preferred upper limit of approximately 1 kHz isdetermined by the time period required for so-called Helmholtzdouble-layers to form on each electrode and in the correspondingadjacent layer of water surrounding an electrode. A Helmholtz doublelayer is essentially a surface charge at each surface of theelectrode-water interface necessary for the reactions of Equation (1)and Equation (2) to occur at the negative and positive electrodes,respectively. As frequency exceeds 1 kHz, there is essentially notenough time for Helmholtz layers to form, and the reactions of Equations(1) and (2) do not occur sufficiently to achieve substantial increasesin intrinsic water ion concentration to effect freezing pointdepression.

Systems and methods in accordance with the invention are useful in awide variety of circumstances and applications to prevent freezing of aliquid water layer and thereby prevent ice formation on the solidsurface. Examples of surfaces that may be protected in accordance withthe invention include, nonexclusively: airplane wings and helicopterblades; windshields and windows of planes, automobiles, and trains; andheat exchanger coils. It is evident that those skilled in the art maynow make numerous uses and modifications of the specific embodimentsdescribed, without departing from the inventive concepts. It is alsoevident that the steps recited may, in some instances, be performed in adifferent order; or equivalent structures and processes may besubstituted for the structures and processes described. Since certainchanges may be made in the above systems and methods without departingfrom the scope of the invention, it is intended that all subject mattercontained in the above description or shown in the accompanying drawingbe interpreted as illustrative and not in a limiting sense.Consequently, the invention is to be construed as embracing each andevery novel feature and novel combination of features present in orinherently possessed by the systems, methods and compositions describedin the claims below and by their equivalents.

1. A system for preventing ice formation on a surface of a solid object,comprising: a first electrode disposed on the surface; a secondelectrode proximate to the first electrode; an interelectrode spaceseparating the first and second electrodes, wherein the interelectrodespace has a thickness not exceeding 3 mm; and an AC power sourceconnected to the first and second electrodes, the power source capableof providing an AC voltage with sufficient power to prevent freezing ofa liquid water layer in the interelectrode space.
 2. A system as inclaim 1, wherein the interelectrode space has a thickness not exceeding500 μm.
 3. A system as in claim 1, wherein the interelectrode space hasa thickness in a range of from 5 nm to 100 μm.
 4. A system forpreventing ice formation on a surface of a solid object, comprising: afirst electrode disposed on the surface; a second electrode covering thefirst electrode, the second electrode being exposed to water and beingporous to water; an interelectrode space separating the first and secondelectrodes; and an AC power source connected to the first and secondelectrodes, the power source capable of providing an AC voltage withsufficient power to prevent freezing of a liquid water layer in theinterelectrode space.
 5. A system as in claim 4, wherein the secondelectrode is a mesh comprising metal mesh fibers.
 6. A system as inclaim 5 wherein the metal mesh fibers have a thickness in a range offrom 1 to 100 μm.
 7. A system as in claim 4, further comprising a porousinsulator layer disposed between the first electrode and the secondelectrode, the porous insulator layer forming the interelectrode spaceand being porous to water.
 8. A system as in claim 7, wherein the porousinsulator layer has a total volume and a pore space, and the pore spaceoccupies between 0 and 100 percent of the total volume.
 9. A system asin claim 8, wherein the pore space occupies in a range of from 50 to 70percent of the total volume.
 10. A system as in claim 7, wherein thefirst electrode comprises aluminum and the porous insulator layercomprises aluminum oxide.
 11. A system as in claim 10, wherein theporous insulator layer comprises anodized aluminum.